Electrical measuring instrument



Dec. 11, 1934. R. D, HlcKoK 1,983,665

ELECTRICAL IVIEASUR] NG T NSTRUMENT Filed Dec. 18,'1931 @An-mehr o l fr J'a i ivf: lfb l?! ,Q4-1 /J a @vh L a1000/ 3h00 /4 35,000 I INVENTOR ATTORNEYS `sistance or dielectric value and hereinafter.

Patented Dec,l ll, 193.4

PATENrfoFFlcE 1,983,665 ELECTRICAL BEASURING INSTRUMENT Robert D. lickok, Cleveland, Ohio,` assigner to Cleveland Patents, Incorporated, Cleveland, mio, a corporation of Ohio Application December 18, 1931, Serial No. 581,886 A i 6 laims. (Cl. 175,-183) This invention'relates to electrical measuring instruments, and particularly to an instrument` designed for the accurate measurement of any' one or more of resistance, capacity and voltage.

Although capable of use for other purposes,1it is particularly designed'for the testing of condensers such asare used in radio receivers and for automobile ignition systems.

Referring particularly to condensers used in ignition systems, such condensers are usually connected in parallel with the primary make and -break. Atthe moment of breaking the primary circuit vconnecting the coil to the battery a high kVoltage is applied across the contacts and is impressed across the condenser, charging the same to a high value, and the discharge of the condenser produces a high potential in the secondary, thus producing the spark for ignition purposes. If the condenser has low dielectric value or, in

other words, if there is a leakage of current through it, it will not charge up to the required high potential and the resulting spark will be weak. It is therefore desirable to test the condenser from two standpoints, to wit, as to its realso as to its ca- Dacty.

In testing such condensers, the application of low voltage is not satisfactory, because under low voltage the condenser may show an apparent high dielectric strength and nevertheless break down or show leakage when high voltage is applied to it,

and` the use of batteries to produce high vVoltage is impractical owing to their size and weight.

The present invention'has for its object to provide an electrical measuring instrument by meansI of which-a condenser or any other device may be tested for resistance under high voltage conditions simulating or approximating those which it is 'intended to encounter in use, and further, an instrument which may be applied 'to the condenser and Y by very simple manipulations may be used for the Further objects of thel invention are in part' obvious and in part will appear more in detail In the drawing, Fig. /1 represents adiagram of .to the instrument.

be arranged to apply any voltage to the con- 1,5 denser, the particular instrument shown in the one simple form of apparatus embodying the invention and Fig. 2 is a similar diagram illustrating another arrangement which is also usable as a voltmeter. Y

Referring first to Fig. 1, which represents an 5 instrument designed for the testing of condensers, but suitable for testing the resistance of any device, the` instrument comprises` an ohmmeter 1 and a capacity meter 2 connected in circuits for alternative use for the purpose of measuring 1 0 either the resistance or the capacity of a condenser subjected to a desired potential as the result of current derived from a source external While the instrument 'may drawing is adapted to apply 300 volts of direct current or 100 volts alternating current to the condenser, the power therefor being derivedfroml anvoutside source, assumed to be a standard 110 20 volt SQ cycle alternating currentsource, the leads to which are indicated at 3. The instrumentembodies a transformer, the primary P of which is adapted for connection to the leads 3- and has in series with it an adjustable regulating resistance 25 R1 of a value of say 250 ohms, and which is utilized for adjusting the applied potential for the purpose of insuring the vapplication to the condenser or other device to be tested of a definite desiredr voltage maintained at the selected Value. transformer also includes two secondaries S and S1, the latter of which is wound to produce the' necessary voltage for a suitable rectier V for producing the direct current which is applied to the condenser. The drawing shows the secondary 35 S1 arranged to produce 5 volts so that either a. standard rectifyin'g tube or any ordinary 5 volt tube may be employed, such as types 201-A, 112-A or the like, the two plates or the plate and the grid thereof being' linkedas shown to operate as one element. 'Ihis arrangement simplifies the apparatus and a'dapts it to readily obtainable commercial tubes.

The secondary S is wound to produce approximately 300 volts potential and is in series with ,a resistance Rz of suitable value, such as 30,000

The 30 'y the condenser to be tested is bridged, This may be accomplished in any suitable manner, such as by direct connection of the condenser plates to the terminals M, or the latter may be provided with leads and prods (not shown) for .application to the condenser. The gap across said terminals may be short circuited by a normally open circuit A including a. normally open push button switch S1, whose purpose will later appear.

Said' terminals M are also connected to a switching device, marked generally S2, of such form as to be capable of operation for selectively or alternately connecting the condenser under test either to the ohrnmeter or to the capacity meter, for direct testing and reading of either its resistance or its capacity. This may be any type oi switch, rotary, swinging or the like, but for convenience is illustrated as a conventional double pole double throw switch with the terminals M connected to its blades B, which operate between two pairs or" contactsC, D, the former of which are in a circuit E in shunt around the continucus load resistance R2 and including the ohmmeter l.

The second pair'of contacts D are in a circuit F in parallel with the primaryP of the transformer and including the capacity meter 2 and a' xed resistance R3 which is chosen of suitable value, say 3200 ohms, according to the maximum capacity to be read upon the capacity meter, and also laccording to the assumed alternating current voltage applied to the primary P, in the present case 100 volts, as well as the sensitivity of the capacity meter 2, which in eiect is an aland condenser C1 is applied directly through the ohmmeter to the condenser to be tested. Theohmmeter is made with a high resistance of approximately 600,000 ohms, which'tenables values as high as 100 megohms to be read directly. The instrument therefore is essentially a so-called megger in its possibility of accurately measuring high resistance.

The-short circuiting switch S1 is used as a test switch andfor the purpose of calibrating the instrument, or in other words, of adjustingthe applied voltage so as to insure the application of exactly 300 volts to the device under test and consequently of causing the pointer of the ohmmeter to travel to full scale position under no load to be tested; In other words, assuming the terminals M connected to a condenser to be tested and the instrument to be connected to the g alternating current source, as shown, the test switch S1 is closed and variable resistance R1 is adjusted until the pointer of the ohmmeter registers accurately at the full scale position.' This A adjustment insures that exactly 3 00 volts will be applied to the condenser under test. The test switch S1 is then released and opens and the pointer of the ohmmeter will move to a position indicating accurately the resistance of the c ndenser under test with a voltage of 300 volts applied to it. The ohmmeter of course is actually a voltmeter -When postssl, 11a are Lacasse calibrated in terms of resistance for use under the circumstances described.

The capacity meter 2 is 'actually anl alternating current milliammeter and while it may be so built as to cover any desired range of capacity, in the present instrument it has been designed to cover a range of capacities from .05 mid. to 5 mfd., the latter value, together with sensitivity and the applied voltage of 100 volts, determining the value ci 3200 ohms for resistance R3. In use of this device the test switch S1 is also employed. In other words, when capacity is to be read, the switch S2 is .thrown to its upper position on capacity contacts D and then test switch S1 is closed, whereupon the variable resistance R1 is adjusted until the pointer of the capacity meter registers accurately at the full scale position, insuring that 100 volts alternating current will be applied -to the condenser under test. The switch S1 is then released and opens, applying the full alternating current voltage to the condenser under test and the capacity meter directly indicates the condenser capacity.

This capacity meter is built and operates acthe fact that if the voltage and frequency in an alternating current circuit are constant, the actual capacityl of a condenser in that circuit may be measured by a suitable alternating current milliammeter calibrated directly in microarads.

Fig. 2 shows a similar instrument designed for 'a wider range of use and also embodying the necessary means enabling it to be used as a voltmeter. The capacity meter 2 is arranged and operates asin Fig. 1.

The meter la. has a single positive post 104 and two negative posts 1 1, 11a and the resistances across the two pairs are proportioned so that when posts 10, 11 are in circuit 400 ohms are lin series and the range is from 0 to 3 volts. in use an extra 3,600 ohms is in series, with a total of 4,000 ohms, and the range is from 0 to 3 volts. Post 10 is connected to a terminal 12 and post 11a to a terminalv 13, the latter being connected through resistance R4 of 36,000 ohms to tap terminal 14, thence through resistance terminal 15 and thence through resistance Re of 400,000 ohms to terminal 16. -With al1 switches open the meter la may-e used as a voltmeter by connecting one lead to terminal 12 and the other to a terminal 13, 14, 15 or 16, with consequent ranges ci 3 volts, 30 volts, 300 volts or 600 volts; The pairs of terminals, of course, will be connected by leads and prods or the like to any two points in a live circuit across which a voltage drop is to be measured, and the meter scale will be graduated with four Rs of 360,000 ohms to,

acording to the well known formula based upon,

diierent setsv of values or indications for its sevan ohmmeter. The secondary S10 is wound to produce'400 volts. The load resistance, marked R2 in Fig. 1, is here tapped into several sections R1, Ra, etc., with a total value of 40,000 ohms and with separate values as indicated. The several sections are connected to a series of pairs of jacks Jo, J1, J2, Ja, J4, connected to the meter 1a and to the several resistances R4, R5 and Rs, as shown.

With this arrangement I use a detachable bridge X normally held in the blank jacks Jo land insertable into any pair of jacks J1, J2, or the like. applied to the device to be tested and the ohmmeter reads from to 30,000 ohms.` In jacks J2 the voltage is 3 and the resistance range from 50 to 300,000 ohms. In'jacks Ja the voltage is 30 and the resistance range from 500 ohms to`3 megohms, while in jacks J4 the voltage is 300 and the resistance range is from 5,000 ohms to 30 megohms, and the meter scale is calibrated accordingly. Switches S1 and IS2 and resistance R1 operate as before.

What I claim is:

1. A combined instrument for testing condensers, comprising an ohmmeter, a capacity meter, circuits adapted for connection to an alshort circuiting said condenser when connectedA to either of said meters.

2. An instrument of the class described, comprising terminals adapted for connection to a condenser to be tested, an ohmmeter, a source of direct current potential therefor, a capacity In jacks J1 .3 voltacross resistance R7 is meter, a source of alternating current potential therefor, switch means adapted to selectively connect said condenser terminals to either said ohmmeter and its source of potential or to said capacity meter and its source of potential, and means for short-circuiting the condenser terminals.

. 3. An instrument of the character described in claim 2, including means for adjusting the potential applied to said ohmmeter.

4. An instrument. of the character described in claim 2, including means for adjusting the potential applied to said capacity meter.

5. An instrument of the class described, comprising a transformer, including a primary adapted'for connection to an alternating current source and provided with means for regulating the applied voltage, the secondary thereof being in a circuit including a rectifier, a condenser and a resistance in parallel relation, whereby definite direct current potential may be applied to said resistance, a circuit in parallel with said resistance and including a meter and means for connecting the same to a device to be tested, means for short circuiting said device to be tested, and means for adjusting the direct current potential applied thereto.

6. An instrument of the class described, comprising a resistance, means for applying a direct current potential thereto, a meter in a circuit in parallel with said resistance, means in said circuit adapted for connection to a device to be tested for determining the resistance thereof, said meter having two posts one of which is provided with tapped external resistance, and separate terminals for connection of said meter in series through selected amounts of said tapped resistance to a device whose voltage is to be tested. ROBERT D. HICKOK. 

